Light-emitting semiconductor devices, such as laser diodes, laser diode arrays, and light emitting diodes (LEDs), are frequently used in conjunction with optical fibers for delivering emitted light to external objects. The efficiency of optical coupling between a semiconductor chip and an optical delivery fiber must be maintained during normal operation of the fiber coupled semiconductor device. The efficiency of optical coupling must also be maintained during assembly and packaging of the fiber coupled semiconductor device, to maximize light generation efficiency of the manufactured device.
A light emitting area of most semiconductor chips is quite small, measuring only a few microns in a direction perpendicular to a plane of thin film layers of the semiconductor device. Generally, small size of the light emitting area is a good, desirable property of a light source, because it is associated with high brightness of the source, allowing the light from the source to be tightly focused, for example. To preserve the brightness of the semiconductor source, it is preferable to use optical fibers having small core diameter. Due to small dimensions of the light emitting area and the small fiber core diameter, the optical fiber has to be precisely aligned to the semiconductor chip. Furthermore, to maintain the emitted power level over a lifetime of the device, the precise alignment between the optical fiber and the semiconductor chip must be maintained over the lifetime of the device.
Referring to FIG. 1, a prior-art fiber-coupled laser diode assembly 10 is shown. The laser diode assembly 10 has been disclosed by Ziari et al. in U.S. Pat. No. 6,758,610 assigned to JDS Uniphase corporation and incorporated herein by reference. The laser diode assembly 10 includes a base 11, a laser chip submount 12, a laser chip 13, a fiber sub-mount 14 including a top section 14A, and an optical fiber 15. The submounts 12 and 14 are affixed to the base 11 with solder layers 16, and the laser chip 13 is affixed to the laser chip submount 12 with the solder layer 16. A solder bead 17 is used to connect the optical fiber 15 to the fiber submount 14. The fiber 15 is metalized to have a metallization layer 18 for improving wettability of the optical fiber 15 by the metal solder material of the bead 17. The top section 14A has low thermal conductivity to serve as a thermal barrier during soldering operation. A front surface 19 of the optical fiber 15 is lensed to improve fiber coupling efficiency.
The optical fiber 15 is aligned to the laser chip 13 using a precision translation stage, not shown. During alignment, the laser chip 13 is energized to produce light, and the optical power of light coupled into the optical fiber 15 is measured. The optical fiber 15 is translated using the translation stage until the coupled optical power is maximized. Then, the melted bead 17 is applied to fix the fiber position. During the cooling down, however, the thermally induced stresses in the submounts 12 and 14, the laser chip 13, and the optical fiber 15 misalign the fiber 15, which results in a loss of some optical power coupled into the optical fiber 15.
Further, disadvantageously, the fiber coupling efficiency of the laser diode assembly 10 is dependent on the ambient temperature even in cases when the laser diode assembly 10 is temperature stabilized using a thermoelectric cooler (TEC). The TEC is not shown in FIG. 1. To remove the heat from the laser diode assembly 10, the base 11 is attached to a top surface of the TEC, and a bottom surface of the TEC is connected to an external heat sink, not shown. When the ambient temperature is different from the temperature of the base 11 of the laser diode assembly 10, the inside and the outside surfaces of the TEC are at different temperatures. The temperature difference results in deformation of the inside TEC surface on which the base 11 is mounted, which causes the base 11 of the laser diode assembly 10 to deform. The deformation of the base 11 results in misalignment of the optical fiber 15 relative to the laser diode chip 13, which results in a reduction of optical fiber coupling efficiency, leading to a reduction of output optical power and the conversion efficiency of the laser diode assembly 10.
Massey in U.S. Pat. No. 7,293,922, which is incorporated herein by reference, discloses a so called “laser hammering” method, which can be used to make fine adjustments of alignment of soldered optical fibers. By way of example, laser hammering can be used to align the fiber 15 to the laser diode chip 13 after cooling down of the solder bead 17, to mitigate the loss of the coupling efficiency due to the thermally induced stresses in the submounts 12 and 14 created upon cooling of the assembly. This approach requires expensive equipment and is limited in its ability to restore the lost coupling efficiency due to somewhat random nature of the effect of the laser pulses.
In U.S. Pat. No. 5,682,453, incorporated herein by reference, Daniel et al. disclose a method of bonding optical elements using glass-based bonding compounds including glass particles and a binding agent. When heat is applied, the binding agent burns away leaving the glass particles fused together. The heat may be applied using a laser. Disadvantageously, a considerable amount of heat needs to be applied locally to melt or at least “soften” the glass particles, so they can be fused together. Local heating is known to create an internal mechanical stress in a system.
In U.S. Pat. No. 6,075,914, incorporated herein by reference, Yeandle discloses an apparatus for connecting an optical fiber to an optical device. In the apparatus of Yeandle, the optical fiber is secured at a location remote from the fiber tip, and the fiber tip is placed in a V-groove to define its position. The heating of the fiber tip is thus avoided. Disadvantageously, this method is limited to passive fiber alignment, which is generally applicable for aligning multimode fibers having a relatively large core diameter.
In U.S. Pat. Nos. 6,734,517 and 7,030,422, incorporated herein by reference, Miyokawa et al. disclose a semiconductor laser diode module constructed so as to reduce the temperature dependence of the fiber coupling efficiency. In the semiconductor laser diode module of Miyokawa et al., the material of a base for supporting a laser chip is selected to match that of a fiber holder. Further, the fiber holder has two parts, one of which is mounted to the base and the other supports the fiber ferrule. The part that is mounted to the base is shaped so that it does not interfere with the laser diode mounting region of the base. Disadvantageously, the module of Miyokawa et al. is rather complex, requiring many laser welding spots to affix all the parts of all the holder elements.
It is a goal of the present invention to provide a simple and inexpensive fiber coupled semiconductor device, in which the fiber coupling efficiency exhibits no substantial decrease both at the aligning/packaging stage and during normal operation. It is also a goal of the invention to provide a method of assembly of such a device.